Plasma arc torches are commonly used for the working of metals, including cutting, welding, surface treatment, melting, and annealing. Such torches include an electrode which supports an arc which extends from the electrode to the workpiece in the transferred arc mode of operation. It is also conventional to surround the arc with a swirling vortex of gas, and in some torch designs it is conventional to also envelope the gas and arc with a swirling jet of water.
The electrodes used in conventional torches typically include an elongate tubular holder composed of a material of high thermal conductivity, such as copper or a copper alloy. One such electrode is disclosed in U.S. Pat. No. 5,023,425 to Severance, Jr., and assigned to the assignee of the present invention. The forward, or discharge, end of the electrode includes an endwall having an emissive insert embedded therein which acts as the cathode terminal for a plasma arc. The insert is composed of a material which has a relatively low work function, defined in the art as the potential step, measured in electron volts, which permits thermionic emission from the surface of a metal at a given temperature. In view of its low work function, the insert readily emits electrons in the presence of an electrical potential. Commonly used insert materials include hafnium, zirconium, tungsten, and alloys thereof.
During operation, the copper holder oxidizes and consequently its work function decreases. Unless discouraged, the plasma arc will soon prefer to attach to the oxidized holder rather than the emissive insert. Once the arc attaches to the holder, the copper oxide and the supporting copper melt and the electrode is rapidly destroyed. To extend the service life of the electrode, an annular sleeve may be introduced to separate the insert from physical contact with the holder. The sleeve is composed of a material resistant to oxidation and having a work function greater than the insert. Because the sleeve resists oxidation and has a relatively high work function, it is a poor emitter. Thus, the likelihood that the arc will transfer from the insert to the sleeve and/or holder is reduced and the useful service life of the electrode is extended.
Nevertheless, a significant problem remains which adversely affects the service life of a plasma arc torch. Specifically, the emissive insert and adjacent portion of the sleeve tend to erode rather quickly. Initially, the surface of the endwall is circular and generally planar. As the torch operates, however, erosion of the insert and adjacent portion of the sleeve leads to the creation of a cavity in the surface of the endwall. In order to attach the insert, the arc must dive into this cavity, thus increasing the arc length. Because plasma is not a perfect conductor, but has a finite conductivity, the elongated arc demands additional voltage. In accordance with Ohms' law, the resistance of the additional length of plasma conductor results in a corresponding voltage drop as the depth of the cavity increases.
The resultant additional voltage is applied between the holder and the insert. The longer the torch operates, the higher the voltage becomes until eventually the additional voltage drop is sufficient to overcome the disadvantage of attachment to the higher work function materials and the arc transfers from the insert to the sleeve and/or holder. The result of double arcing from the sleeve and/or holder is rapid erosion of the electrode which can lead to double arcing and rapid destruction of the nozzle-electrode pair. It is also believed that during operation, erosion products from the insert are deposited on the sleeve and holder, thus reducing their work functions and increasing the likelihood of double arcing.
It is accordingly an object of the present invention to provide an electrode and method of operating a plasma arc torch which improves the useful service life of the torch.
It is another object of the present invention to provide an electrode for a plasma arc torch which reduces the likelihood of the occurrence of double arcing, thereby preventing destruction of the nozzle-electrode pair.
It is a more particular object of the present invention to provide an electrode having channels formed therein for directing a portion of a swirling gas into the cavity created in the endwall of the electrode by erosion of the insert and adjacent portion of the sleeve, thereby widening the cavity and reducing the additional voltage drop at the electrode. Furthermore, this directed flow of the gas will remove the erosion products from the sleeve and the holder making these parts less conducive to arcing.